1. Field of the Invention
The present invention relates to a workpiece fastening device which presses a plurality of locations of a workpiece to fasten the workpiece.
2. Description of the Related Art
There is a device known in the art, which presses a plurality of locations of a workpiece to fasten the workpiece. For example, JP H11-311240A discloses a workpiece clamp fixture which has a pressing member which is connected to a piston rod, and presses a plurality of locations on a larger end part of a connecting rod by means of first to third pressing parts which are integrally formed with the pressing member. In addition, in the field of machine tools, a workpiece fastening device which presses a plurality of locations on a workpiece has been used for stably fastening a workpiece to be processed.
FIG. 5 is a perspective view which shows one example of the above-mentioned conventional workpiece fastening device. The workpiece fastening device 5 of FIG. 5 has four air cylinders 53 which are arranged so as to surround a table 51 of a workpiece W, and pressing arms 52 which are connected to piston rods 532 of the air cylinders 53. In the workpiece fastening device 5 of FIG. 5, when the air cylinders 53 operate and then the arms 52 as a whole move downward in the vertical direction, the front end parts of the arms 52 press the corresponding locations on the workpiece W (pressed parts P) against the table 51. The workpiece fastening device 5 of the present example is provided with the same number of (four) air cylinders 53 as the pressed parts 5 of the workpiece W, and therefore it is necessary to secure sufficient installation space around the table 51.
FIG. 6 is a perspective view which shows another example of a conventional workpiece fastening device. The workpiece fastening device 6 of FIG. 6 is used for fastening a workpiece W of the same shape as FIG. 5. However, the workpiece fastening device 6 only has two pressing arms 62 which are arranged at the opposite sides of the table 61, unlike the workpiece fastening device 5 of FIG. 5. More specifically, the workpiece fastening device 6 of this example has a structure where the front end parts of the arms 62 are branched into two parts, and is configured to press the pressed parts P of the workpiece W by means of the pressing parts 621, 622 which are formed on these branch parts. If the workpiece fastening device 6 of FIG. 6 is used, it is possible to reduce the numbers of the air cylinders 63 and arms 62, and therefore it is no longer necessary to secure a large installation space around the table 61.
On the other hand, when the workpiece fastening device 6 of FIG. 6 is used, the arm 62 is liable to apply uneven loading to the workpiece W due to manufacturing error of the workpiece W. FIG. 7 is a schematic view for explaining the above uneven loading which may occur in the workpiece fastening device 6 of FIG. 6. More specifically, FIG. 7 is a side view of one arm 62 in FIG. 6 as seen from the position of the other arm 62. In FIG. 7, for convenience, the outside shape of the workpiece W which is placed on the table 61 is shown by the broken lines. Here, the workpiece W in FIG. 6 and FIG. 7 is designed so that the surfaces of the four pressed parts P are positioned on the same plane, and the two pressing parts 621, 622 of each arm 62 are arranged side by side in the horizontal direction so as to be able to simultaneously abut against the surfaces of two pressed parts P. However, if the surfaces of these pressed parts P are not positioned on the same plane due to manufacturing error of the workpiece W, only one of the two pressing parts 621, 622 will abut against and press the workpiece W.
In the example of FIG. 7, the surface of the left side pressing part P in the figure is positioned higher than the surface of the right side pressed part P in the vertical direction. For this reason, if the air cylinder 63 moves the arm 62 downward, only the left side pressing part 622 in the figure will abut against and press the workpiece W. The movement direction of the arm 62 in this process is shown by the arrow A70 in the figure. If the surfaces of pressed parts P of the workpiece W are not positioned on the same plane, these pressed parts P are not evenly pressed by the pressing parts 621, 622 of the arm 2, and therefore the arm 62 as a whole may apply uneven loading to the workpiece W. If the above uneven loading occurs in the machine tool, there is a possibility of deformation of the workpiece W due to the pressing forces of the individual pressing parts 621, 622 or deterioration of the processing precision due to insufficient fastening of the workpiece.